The impact of a cosmic time evolution of the gravitational
constant on SN Ia luminosity and AGN/QSO luminosity functions is studied. The
gravitational constant scales linearly with the Hubble parameter, its
present-day variation being dot{G}0/G0≈ 1.9×10^{-4} Gyr‑1,
compatible with current bounds from lunar laser ranging. Distance moduli of
Type Ia supernovae are fitted with a cosmic expansion factor derived from
temperature variations of planetary paleoclimates, and a luminosity dependence
on look-back time proportional to the varying gravitational constant is
inferred from the Hubble diagram. A fit is performed to the comoving space
density of X-ray-selected active galactic nuclei (AGNs) and optically selected
quasars (QSOs) extending to redshifts z≈6. The initial steep increase of the
AGN space density is reproduced by a redshift evolution depending solely on the
Hubble parameter as scaling variable. The AGN luminosity scales with the Hubble
parameter, and the scaling exponents of the luminosity function, composed of
two competing power laws with exponential cutoff, are obtained. Based on the
AGN luminosity function, flux-limited X-ray source counts are investigated. The
counting functions are derived and put to test by fitting cumulative number
counts of soft X-ray point sources compiled from ROSAT, XMM-Newton, and Chandra
surveys.